Fibre-reinforced Positioning Lever for Manufacturing Equipment
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KeywordsBase Body Interface Element Hybrid Construction Packaging Machine Position Lever
When developing manufacturing equipment, short cycle times are the key issue. Elastic deformation of working bodies can limit the top speed of such systems. Since the deformation is caused by the acceleration of these components, the mass and stiffness of the moving parts are crucial. CFRP offer significant advantages over metallic materials in this respect.
The use of ultra-modern processing machines is one of the key requirements for a globally interconnected consumer society. In addition to plastic, paper, printing and other special machines, packaging machines in particular are a key component for the provision of commodities and consumables. While reliability and ecological aspects are important, the working speed of the systems deployed is the most important criterion. In recent years, productivity requirements coupled with ever-intensifying market competition have led to a significant reduction of the required cycle times for designing such processing machines. Despite primarily non-uniform movement routines of working bodies of packaging machines, certain applications achieve speeds of more than 1000 cycles per minute [1, 2]. However, there is very little scope for even greater productivity given the inherent material properties of the metallic substances used.
The use of more modern materials thus seems one option to boost performance. Due to a combination of high stiffness, low density and considerable damping capacity fibre-reinforced plastics (FRP) are already deployed in many highly dynamically configured constructions. However, while the process of continuous filament reinforcement is already applied to plastic for use in highly stressed components deployed in a range of industries, its potential has been scarcely exploited in the packaging machine sector . In order to tap into this potential to boost efficiency, Leichtbau-Zentrum Sachsen (LZS), supported by the Institute for Lightweight Engineering and Polymer Technology (ILK) of the Technische Universität Dresden has developed a fibre-reinforced positioning lever for use in packaging machines.
In addition to mechanical needs imposed, the conceptual development of new components also has to meet additional requirements such as mounting accuracy and installation space. Positioning levers intended for use in the food processing industry are subject to specific requirements for hygiene and safety for human health of the materials used [4, 5].
To prevent any contamination of foodstuff, any corners in which production residues could accumulate must be eliminated early on in the design stage. If critical areas cannot be avoided for design-related reasons (for example, in the area of joints when positioning levers feature a hybrid construction), these must be either covered by additional components or designed so as to be easily accessible and thus easy to clean. In accordance with Regulation No. 10/2011 of the European Commission, additional specific provisions apply to fibre composite components (“multilayer composites”), which could come into contact with foodstuffs during the handling process.
To guarantee the safety of the materials used for human health, epoxy-resin based polymers including a certified food-safe coating must be used. This coating process involves applying a colourless homogeneous surface treatment to CFRP components, which makes it easier to detect any contamination or damage. In accordance with current legislation, the status of materials as safe for use with foods is also regulated in standards, for example DIN EN 1672-2: Food processing machinery — Basic concepts — Part 1: Safety requirements and Part 2: Hygiene requirements.
The loads exerted from joints and actual operation impose a complex stress pattern on the base body, which prompts the use of isotropic materials. Moreover, the close proximity to the rotation axis reduces the influence of the base body mass on the overall load-bearing capacity of the component. The lever arm, conversely, given its centre of gravity far away from this axis, is the primary element determining the inertia of the positioning lever while in operation and the resulting process-critical deformation.
Using a lighter and more rigid material is key in this field. The lever arm itself is mainly subject to bending and torsional stresses while in operation. The result — a relatively simple stress state — paves the way to optimally exploiting the strongly anisotropic rigidity properties of CFRP materials. To obtain the targeted resisting moment against bending and torsional stresses with a low component mass, the structure of the lever arm should preferably be a sealed hollow profile cross-section. This design also reduces the accumulation of residues.
▸ winding followed by resin-transfer moulding (RTM) method
▸ autoclaving of correspondingly shaped prepregs
▸ pressing of organic sheets.
Specific advantages and disadvantages of selected production methods for manufacturing CFRP components (© LZS)
+ uncomplicated function
+ good mechanical properties
Winding + RTM
+ partially automatable
− cost-intensive prototype production
− limited shelf angle when winding
+ uncomplicated functional integration
+ good mechanical properties
Prepreg + Autoclave
+ low production costs for prototypes
− difficult pressing of individual layers
− high proportion of manual procedures
+ inexpensive materials
+ short cycle times
Organic sheet + Presses
− very cost-intensive prototype production
− high development expense
− lower lightweight limit
When designing components for use in the food processing industry, flat surfaces free of corners and edges in which dirt can accumulate are required . Fibre-reinforced hollow sections that meet this requirement can either be produced by gluing together two individually manufactured half-shells or using the film bubble process. Given the small size of the lever arm being produced in this case, a glued joint would prove disadvantageous, since it would lead to a significant decline in torsional strength.
As well as the advantage of uniform and easily adjustable compressive force via air pressure, this approach also eliminates the need for an autoclave. The entire process can be performed in a heating chamber. Unlike conventional vacuum structures, the silicone tube is reusable.
The individual components of the positioning lever are dimensioned in accordance with stiffness requirements. The design aims to configure a component with working elements which are subject to as little deformation as possible while in operation. The vast majority of the load exerted does not occur as a result of the impact from the packaged item, but as a result of the very frequent acceleration of the component in question. The displacement simulated in the finite elements (FE) model at the interface element mainly comprises bending and torsion of the lever arm. The layer structure of the CFRP components is optimised numerically to minimise the level of deformation.
In one further FE model, an eigenfrequency analysis is performed while using an optimised layer structure. When the system-specific eigenfrequencies are compared with the excitation frequencies, the speeds at which resonances may occur can be determined. Where applicable, adjusting the laminate structure and the associated change in rigidities can prevent upward surging of the structure in the desired speed ranges.
The DFT proves that proportions of the harmonic frequencies above sixfold base frequency are only included in the excitation signal to a marginal extent. Due to the fact that the first eigenfrequency in the output range of up to 1000/min only intersects with the 18th harmonic, resonances are unlikely to occur while the positioning lever is in normal operation. Additional numeric investigations of the resonance behaviour confirmed this finding.
This hybrid application also boosts the cycle time by 143 % compared to conventional Al construction.
Conclusion and Outlook
To allow these simulation results to be validated, a prototype of the hybrid positioning lever will be manufactured. Motion measurements and a permanent operation test are going to be performed on a test rig. The results of the planned experimental investigations will be used for further modifications of the FE models.
The authors would like to thank engineers Felix Dehmel, Stefan Hoschützky and Achim Mertel for assisting with this article.
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